1. Field of the Invention
This invention relates to RF power amplifiers, and particularly to spatially combined power amplifiers for high frequency applications in the EHF-band and above.
2. Description of the Related Art
Obtaining high power (>1 W) using solid-state amplifier chip technology at Extremely High Frequencies (EHF) (110-300 GHz) and particularly >200 GHz is widely viewed as improbable or impossible using today's technology. With today's technology each amplifier chip can output tens of mW of power in the EHF band. Therefore, one must combine the output power from several amplifier chips to provide >1 W of output power.
Ideally, the output powers of the amplifier chips would combine linearly. This assumes no combining loss (no conductor losses or phase/amplitude errors that effect constructive/destructive adding of power). This is obviously not practically achievable. Achieving high output power is not as simple as adding more amplifier chips to obtain the desired power level.
The typical combining approaches used across the industry include corporate or radial combining networks. The main limitation is that as more and more amplifier chips are added to the system to combine power, there is a point of diminishing returns where as more chips are added, the power will stop increasing, and will actually begin to decrease. This is mostly due to added conductor losses associated with increased distances in the network (more elements means more physical splits and greater length in the lines between each split). This problem is exacerbated at the higher frequencies in the EHF band. It is not realistically possible nor practically feasible to obtain >1 W of output power using today's amplifier chip technology in the EHF band.
Spatial combining is as close to ideal as you can get because there is no physical structure (corporate or radial) combining the power of all the elements. The power is combined in free space, so there are minimal conductor losses to deal with, and small phase and amplitude errors have less effect on the total array combining efficiency. Spatial combining has been successfully employed at lower frequencies in the W-band (75-110 GHz) to provide combined output powers >1 W. See U.S. Pat. No. 8,107,894 entitled “Modular Solid-State Millimeter Wave (MMW) RF Power Source” issued Jan. 31, 2012 and assigned to Raytheon Company, which is hereby incorporated by reference. However, the same design is inadequate at frequencies in the EHF band due to increased losses; the design simply does not scale to the higher frequency band.